In prior art ground proximity warning systems the warning criteria or warning envelopes have been normalized to provide a practical balance between giving a pilot timely warning when the aircraft is in a situation where inadvertent flight into terrain is a possibility while at the same time resulting in a minimum of nuisance warnings. A nuisance warning is a warning generated by a ground proximity warning system when the aircraft is operating normally with respect to the terrain and there is little or no danger of inadvertently flying into the ground. Nuisance warnings are, however, considered to be highly undesirable in that they tend to reduce pilot confidence in the warning system and may result in the pilot disregarding a subsequent valid ground proximity warning. As a result it has always been considered highly desirable to minimize nuisance warnings to the maximum extent compatible with providing timely warnings where the aircraft is actually in some danger of impacting the ground. Heretofore attempts to reduce nuisance warnings have centered primarily on the attempt to provide optimum warning envelopes for each type of terrain encountered by the aircraft such that one set of warning envelopes would provide a balance between nuisance warnings and actual warnings worldwide. Illustrations of prior art attempts to provide normalized warning envelopes or criteria for all flight situations are provided in U.S. Pat. Nos.
______________________________________ 3,715,718 3,934,221 3,946,358 3,925,751 3,958,218 3,934,222 3,922,637 4,030,065 3,944,968 4,060,793 3,947,809 4,215,334 3,947,808 4,319,218 3,947,810 ______________________________________
As a result of extensive studies of ground proximity warning systems in commercial use throughout the world, it has been discovered that there are instrument approaches to certain airports where the terrain along the approach path may be such that the warning time in the case of an actual ground proximity warning is not optimum if the airport is located at a relatively high altitude with respect to the surrounding terrain. An example of this type of situation is the instrument landing system approach to runway 24 at Hot Springs, Va., where the terrain underlying the approach climbs rapidly up to the runway elevation. Consequently, the aircraft might not reach the radio altimeter arming height for the glideslope mode until the aircraft is within one-half nautical mile of the runway threshold which may be too late for recovery of the aircraft if it has inadvertently descended below the runway elevation. Similarly there are a number of airports where because of the underlying terrain, the approaches or departures may result in an undesirable number of nuisance-type warnings. These airport approaches or departures include runway 13 at Hong Kong, runway 15 at Leeds Bradford, United Kingdom, runway 26R at Ontario, Calif. and runway 26 at Victoria, British Columbia.